miércoles, 17 de abril de 2013

You don’t just strap a satellite to a rocket,
launch it, and voilà, it takes measurements. Beyond maneuvering into
the right orbit, there are a series of check-out procedures to make sure
the satellite performs in space as it did in ground tests. You have to
make sure the communication signals are strong and clear. You have to
exercise moving parts like shutters and doors. You have make sure the
solar panels are oriented to the Sun and batteries and thrusters are
working properly.For a satellite designed to take images—such as the Landsat Data Continuity Mission
(LDCM)—a critical step is calibration, both at the beginning of the
mission and throughout its lifetime. There are many steps to
calibration, but a key one is determining whether the observations of
light and color reflected by Earth’s surface match what can be observed
with instruments and eyes on the ground. And since LDCM is the eighth satellite in a long line of Earth observers, calibration also involves matching new observations with how the previous satellites saw things.The natural-color satellite image above was captured by the
Operational Land Imager on LDCM on March 29, 2013. The scene includes a
dry lake bed in the Arizona desert known as Red Lake Playa. On the day
of the satellite image, researchers took measurements with instruments
on the ground and from an airplane (second image) while LDCM took
measurements from about 700 kilometers (500 miles) overhead. LDCM flew
on the same orbital path (but slightly lower altitude) as the Landsat 7
satellite, which has been making observations since 1999. The two satellites took coincident measurements from March 29–31, collecting more than 1,000 common scenes.Like the charge-coupled device on your digital camera, OLI turns
measurements of light reflected by Earth’s surface into voltages that
can be digitally stored and transmitted. Researchers compare those
measurements with what was observed on the ground, making small
adjustments that are similar to adjusting the brightness on a digital
photograph. It is important for instrument performance to be consistent
across different generations; if not, differences between the images
could be misunderstood as changes in the landscape. This means the LDCM
team must not only make sure the satellite compares well with previous
Landsats, but they must also re-evaluate old images as new technologies improve the accuracy of the view.The playa in Arizona—not far from the towns of Dolan Springs (left)
and Kingman (not in view)—was chosen for the calibration activity
because it is remote, sparsely vegetated, high altitude, and flat. “We
like to use bright, uniform and level sites, often in the southwestern
United States” said Brian Markham, the leader of the LDCM calibration
team and a scientist at NASA’s Goddard Space Flight Center. Such
features assure that there is less distortion of the signal by moisture,
clouds, and pollution in the atmosphere. Such areas also tend to change
little over time, making them ideal for calibrating instruments over
several generations.In the photo, NASA Goddard scientist Joel McCorkel is operating the
Solar and Lunar for Absolute Reflectance Imaging Spectrometer (SOLARIS),
a portable instrument to observe the properties of light reflected by
the land surface. The plane is carrying the Goddard Lidar, Hyperspectral, and Thermal
(G-LiHT) instrument, a portable imaging system that maps the
composition, structure, and function of terrestrial ecosystems. “The
lidar provides 3D information about the distribution of foliage and
canopy elements,” said Bruce Cook, also from Goddard. “Imaging
spectroscopy helps discern species composition and variations in
biophysical variables. And the thermal measurements quantify surface
temperatures and detect heat and moisture stress.”“These measurements transfer the advanced laser-based calibration
from the lab to the test site and finally to the new and old Landsat
sensors,” said McCorkel. “The ground and airborne instruments used
during this campaign measured light in hundreds of separate spectral
(color) channels. This means that effects of differences between the new
and old Landsat sensors can be studied in greater detail.”

Satellite image by Robert Simmon, using Landsat
data from the U.S. Geological Survey and NASA. NASA photo by Jason
Hair, NASA Goddard Spaceflight Center. Caption by Mike Carlowicz.